As you say, ken, the OEM product can only be installed on the system it is licensed for. A retail version can be installed on a desktop and a portable (laptop etc) for non-concurrent use by the licensed user, or someone they are allowing to use the software.
The Office license is a user license, in contrast to an Operating System which is a machine license. The OEM version is further restricted by being a cheaper one only valid for the one machine it is installed on.
The retail version costs more but one of its advantages is you are allowed to also have it installed on the portable computer, but only use one or the other at any one time, not both together (because it is only one user license). If you want two people to be able to use Office on that desktop and the laptop at the same time, you should have a copy of Office for each computer.
Another advantage of the retail version is it can be removed from one computer and re-installed on another, eg when upgrading to a new computer.http://office.microsoft.com/en-us/products...2103171033.aspx